1. Field of the Invention
The present invention is related to mobile communications and, more particularly, to increasing existing base station capacity without requiring additional antennas.
2. Background Description
Wireless communication systems, such as those supporting Global System for Mobile Communication (GSM), Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA) technologies, are characterized by cells or cell sectors with a base transceiver station (BTS) in each. Each BTS, also commonly referred to simply as a “base station,” has one or more antennas to support wireless communications with local mobile units, each on a corresponding one of a given number (X) of carriers. Typically, any of the base station antennas can handle mobile units on any of the X carriers. At the antenna and between the particular mobile unit and the antenna, downlink or transmit signals (Tx) are duplexed with uplink or receive signals (Rx) to share a common carrier frequency. A duplexer in the base station separates/combines the channel signals, essentially blocking the outgoing or downlink transmit signal to prevent it from returning to a connected receiver at the incoming or uplink side.
As cell phone technology has become more popular, the number of mobile units in use at any one time has increased proportionately. Thus, the number of mobile units that each base station must handle simultaneously has correspondingly increased. However, keeping pace with this ever-increasing demand has been difficult and very expensive.
If space is available and local authorities are amenable, the most straightforward way to increase wireless network capacity is simply to add more antennas, either by adding more base stations or by adding antennas to existing base stations. Each additional antenna increases the number of carriers available for the base station and, correspondingly, the number of mobile units the base station can handle. However, even with community approval, this is very expensive.
Alternately, capacity may be increased by inserting a standard hybrid combiner stage between an existing antenna and base station duplexers to combine the duplexed carriers for the particular antenna. While a typical state of the art hybrid combiner stage may serve to boost the number of carriers that a base station can handle, the carrier capacity increase does not come without a penalty. Each hybrid combiner stage reduces path signal strength in both directions between the antenna and base station by half or more, i.e., reduces by at least 3 dB both downlink power at the antenna and uplink power to the base station. While an amplifier may be used to amplify the uplink signal at the antenna (and before the hybrid combiner) to recover uplink power and minimize the resulting impact on Rx sensitivity, downlink power can only be recovered by increasing base station transmitter power. Consequently, in cells serviced by downlink-limited base stations, mobile units at the original cell perimeter no longer receive service as a result of increasing base station carrier capacity without increasing transmitter power. This is because losing half of the duplex signal at the duplexer reduces base station coverage for the cell, i.e., to a quarter or less of its original area. So, to maintain coverage when increasing transmitter power is not feasible, base station sites must be added to the network to fill in any remaining coverage holes. Unfortunately, both of these solutions to recovering the signal lost by introducing hybrid combiners are accompanied by serious disadvantages.
Both consume resources that add to base station/network costs including added equipment, installation and operating costs, and complicated network support and maintenance. Adding expensive components to increase downlink power further increases the footprint of each base station and increases base station energy consumption. Also, since each new component has a quantifiable failure rate that combines non-linearly with that of other components, adding these components reduces site reliability. Each new base station site adds not only equipment costs, but also requires real property purchases and construction costs. Furthermore, adding a base station site may require an appreciable effort to obtain the necessary local/national approvals, regulatory and otherwise.
Thus, there is a need for a way to increase existing network capacity with minimal negative impact to the existing network.